(1) Field of the Invention
The present invention relates to a battery feed circuit, more particularly, to a battery feed circuit used as a constant-current battery feed circuit for subscriber lines in a switching system.
(2) Description of the Related Art
As well known, battery feed circuits play an important role in feeding a constant DC current, via subscriber lines, to telephone sets, data terminals, and other subscriber units. Battery feed circuits are thus some of the most important parts in a switching system. Recently, the need has been growing for switching network featuring increased quality of transmission signals, decreased power consumption, reduced systems costs, and so on. The need is being met by improvements in hardware, including battery feed circuits.
There are several types of battery feed circuits, e.g., a "constant-resistor" type and a "constant-current" type. Constant-resistor types are undesirable from the viewpoint of power consumption. Constant-current types feature lower power consumption, but suffer from the problem of instability and undesired clipping of transmission signals due to common mode noise induced along the subscriber lines.